Various lock designs are found which utilize the principles of the aforedescribed arrangement. For example, U.S. Pat. No. 1,070,367 (Voight) describes a cylinder lock having an additional functional position for a special key. When the plug occupies this additional functional mode, access can be had to the apartment or room, solely with the aid of this special key. In the preferred embodiment, an upper pin located in a pin-channel in the row of channels in the additional functional mode has an upwardly extending peg, which causes the pin to engage the roof of the plug housing, thereby to prevent the plug from being turned with any key other than the aforesaid special key, when the plug occupies with additional functional position.
A lock of this design is unsatisfactory, however, since it is a relatively simple matter to file a substitute key so that the part of the key co-acting with the studded pin is able to move the same in a manner to enable the plug to be turned. Thus, a standard key can readily be converted to the aforesaid special key, and be used to gain entry to the apartment, even when the lock is turned to the additional functional mode.
Published Norwegian patent application No. 793880 (Elkem-Spigerverket) describes a similar arrangement in which the number of pin-channels in the normal lock position differ from the number of pin-channels operative in the additional functional mode or service mode thereof, thereby enabling the key to be locked. Blocking of the key is effected by excluding the presence of certain pin-channels for upper pins in one of the said lock positions, so as to prevent the lower pin or pins from moving upwardly in this position. This lock also has the same disadvantage as the lock previously described, since it is a simple matter to modify a standard key to fit the lock and function in the additional or service mode, thereby overriding the design latching effect.
U.S. Pat. No. 4,300,374 (Mullich et al) describes a similar lock arrangement, although in this case the special key has limited manouverability.
U.S. Pat. No. 1,922,438 (Hurd) describes an arrangement in which pin-sections can be tipped over in a manner to retain the key.
One disadvantage with the majority of the aforementioned known lock designs is that they can readily be forced. This applies, for example, to the described Norwegian lock design, when one has pre-knowledge of the particular pin-channel which does not co-act with a pin-channel in the plug housing. Another disadvantage is that the service key can readily be filed to fit the lock in its normal lock position.